Acrylic acid, acrylic acid derivatives, or mixtures thereof are used today in a variety of industrial materials, such as adhesives, binders, coatings, paints, polishes, detergents, flocculants, dispersants, thixotropic agents, sequestrants, and superabsorbent polymers (SAP), which are used in disposable absorbent articles, including diapers and hygienic products. In terms of production process, acrylic acid is typically made today from the two-step catalytic oxidation of propylene, which in turn is produced from fossil resources, such as petroleum or natural gas. More on the oxidation of propylene to make acrylic acid and other production methods can be found in the Kirk-Othmer Encyclopedia of Chemical Technology, Vol. 1, pgs. 342-369 (5th Ed., John Wiley & Sons, Inc., 2004).
Fossil-derived acrylic acid contributes to greenhouse emissions due to its high content of fossil-derived carbon. Furthermore, the fossil resources are non-renewable, as it takes hundreds of thousands of years to form naturally and only a short time to consume. On the other hand, renewable resources refer to materials that are produced via a natural process at a rate comparable to their rate of consumption (e.g., within a 100-year time frame) and can be replenished naturally or via agricultural techniques. Examples of renewable resources include plants, such as sugar cane, sugar beets, corn, potatoes, citrus fruit, woody plants, lignocellulose, carbohydrate, hemicellulose, cellulosic waste, animals, fish, bacteria, fungi, and forestry products. As fossil resources become increasingly scarce, more expensive, and potentially subject to regulations for CO2 emissions, there exists a growing need for non-fossil-derived acrylic acid, acrylic acid derivatives, or mixtures thereof that can serve as an alternative to fossil-derived acrylic acid, acrylic acid derivatives, or mixtures thereof.
Many attempts have been made over the last 80 years to make non-fossil-derived acrylic acid, acrylic acid derivatives, or mixtures thereof from renewable resources, such as lactic acid (also known as 2-hydroxypropionic acid), lactic acid derivatives (e.g. alkyl 2-acetoxy-propionate and 2-acetoxy propionic acid), 3-hydroxypropionic acid, glycerin, carbon monoxide and ethylene oxide, carbon dioxide and ethylene, and crotonic acid. From these resources, only lactic acid is produced today in high yield and purity from sugar (≧90% of theoretical yield, or equivalently, ≧0.9 g of lactic acid per g of sugar), and with economics which could support producing acrylic acid cost competitively to fossil-derived acrylic acid. As such, lactic acid or lactate presents a real opportunity of serving as a feedstock for bio-based acrylic acid, acrylic acid derivatives, or mixtures thereof. Also, 3-hydroxypropionic acid is expected to be produced at commercial scale in a few years, and as such, 3-hydropropionic acid will present another real opportunity of serving as feedstock for bio-based acrylic acid, acrylic acid derivatives, or mixtures thereof. Sulfate salts; phosphate salts; mixtures of sulfate and phosphate salts; bases; zeolites or modified zeolites; metal oxides or modified metal oxides; and supercritical water are the main catalysts which have been used to dehydrate lactic acid or lactate to acrylic acid, acrylic acid derivatives, or mixtures thereof in the past with varying success.
For example, U.S. Pat. No. 4,786,756 (issued in 1988), describes the vapor phase dehydration of lactic acid or ammonium lactate to acrylic acid using aluminum phosphate (AlPO4) treated with an aqueous inorganic base as a catalyst in a Pyrex reactor. The '756 patent discloses a maximum yield of acrylic acid of 43.3% when lactic acid was fed into the reactor at approximately atmospheric pressure, and a respective yield of 61.1% when ammonium lactate was fed into the reactor. In both examples, acetaldehyde was produced at yields of 34.7% and 11.9%, respectively, and other side products were also present in large quantities, such as propionic acid, CO, and CO2. Omission of the base treatment caused increased amounts of the side products. Another example is Hong et al., Appl. Catal. A: General 396:194-200 (2011), who developed and tested composite catalysts made with Ca3(PO4)2 and Ca2(P2O7) salts with a slurry-mixing method. The catalyst with the highest yield of acrylic acid from methyl lactate was the 50%-50% (by weight) catalyst. It yielded 68% acrylic acid, about 5% methyl acrylate, and about 14% acetaldehyde at 390° C. in a Pyrex reactor. When the feed changed from methyl lactate to lactic acid, the same catalyst achieved 54% yield of acrylic acid, 14% yield of acetaldehyde, and 14% yield of propionic acid. Prof. D. Miller's group at Michigan State University (MSU) published many papers on the dehydration of lactic acid or lactic acid esters to acrylic acid and 2,3-pentanedione, such as Gunter et al., J. Catalysis 148:252-260 (1994); and Tam et al., Ind. Eng. Chem. Res. 38:3873-3877 (1999). The best acrylic acid yields reported by the group were about 33% when lactic acid was dehydrated at 350° C. over low surface area and pore volume silica impregnated with NaOH in a Pyrex reactor. In the same experiment, the acetaldehyde yield was 14.7% and the propionic acid yield was 4.1%. Examples of other catalysts tested by the group were Na2SO4, NaCl, Na3PO4, NaNO3, Na2SiO3, Na4P2O7, NaH2PO4, Na2HPO4, Na2HAsO4, NaC3H5O3, NaOH, CsCl, Cs2SO4, KOH, CsOH, and LiOH. In all cases, the above referenced catalysts were tested as individual components, not in mixtures. Finally, the group suggested that the yield to acrylic acid is improved and the yield to the side products is suppressed when the surface area of the silica support is low, reaction temperature is high, reaction pressure is low, and residence time of the reactants in the catalyst bed is short. Finally, the Chinese patent application 200910054519.7 discloses the use of ZSM-5 molecular sieves modified with aqueous alkali (such as NH3, NaOH, and Na2CO3) or a phosphoric acid salt (such as NaH2PO4, Na2HPO4, LiH2PO4, LaPO4, etc.). The best yield of acrylic acid achieved in the dehydration of lactic acid was 83.9%, however that yield was achieved at very long residence times.
Therefore, the manufacture of acrylic acid, acrylic acid derivatives, or mixtures thereof from lactic acid or lactate by processes, such as those described in the literature noted above, has demonstrated: 1) yields of acrylic acid, acrylic acid derivatives, or mixtures thereof not exceeding 70% at short residence times; 2) low selectivities of acrylic acid, acrylic acid derivatives, or mixtures thereof, i.e., significant amounts of undesired side products, such as acetaldehyde, 2,3-pentanedione, propionic acid, CO, and CO2; 3) long residence times in the catalyst beds; 4) catalyst deactivation in short time on stream (TOS); and 5) operations in Pyrex reactors. The side products can deposit onto the catalyst resulting in fouling, and premature and rapid deactivation of the catalyst. Further, once deposited, these side products can catalyze other undesired reactions, such as polymerization reactions. Aside from depositing on the catalysts, these side products, even when present in only small amounts, impose additional costs in processing acrylic acid (when present in the reaction product effluent) in the manufacture of SAP, for example. These deficiencies of the prior art processes and catalysts render them commercially non-viable.
Accordingly, there is a need for methods of making acrylic acid, acrylic acid derivatives or mixtures thereof from hydroxypropionic acid, hydroxypropionic acid derivatives, or mixtures thereof with high yield, selectivity, and efficiency (i.e., short residence time); high longevity catalysts; and in industrially-relevant reactors with low corrosion rates.